A liquid concentration detecting device including a first substrate, a first temperature sensing element and a concentration sensor is provided. The first temperature sensing element and the concentration sensor are respectively disposed on opposite first surface and second surface of the first substrate. The concentration sensor includes a second substrate, a porous element, a heating element and a second temperature sensing element. The second substrate is disposed above the second surface. A portion of the liquid flows into the concentration sensor through the porous element, and the heating element heats the liquid in the concentration sensor. The second temperature sensing element measures the temperature variation of the liquid in the concentration sensor. The measured temperature and the temperature variation are compared to deduce a concentration of the liquid under detection.